I would like to backup using time machine WITHOUT reformatting external drive...

I have a MacBook that I'm trying to backup because my husband wants to sell my computer and buy an iPad I have a WD external hard drive with an iPhoto library currently on it, chronicling the first 28 months of my first child's life; needless to say it's important!

The problem is that Time Machine won't back up to the external drive without erasing everything on it! Is there any way around this?

I will probably also need tips on how to put the iPhoto library (28 months to present time, about 2,200 pictures!) on my MacBook onto the hard drive (does that automatically happen upon backup?). Obviously I've done it once, because there is a library currently on the hard drive, but I can't remember how I did it.

Alright, hit me with some ideas! Thanks!